[c#] Adding new line of data to TextBox

I'm doing a chat client, and currently I have a button that will display data to a multi-line textbox when clicked. Is this the only way to add data to the multi-line textbox? I feel this is extremely inefficient, because if the conversation gets really long the string will get really long as well.

private void button1_Click(object sender, EventArgs e)
        {
            string sent = chatBox.Text;
            displayBox.Text += sent + "\r\n";

        }

I find this method saves a lot of typing, and prevents a lot of typos.

string nl = "\r\n";

txtOutput.Text = "First line" + nl + "Second line" + nl + "Third line";

C# - serialData is ReceivedEventHandler in TextBox.

SerialPort sData = sender as SerialPort;
string recvData = sData.ReadLine();

serialData.Invoke(new Action(() => serialData.Text = String.Concat(recvData)));

Now Visual Studio drops my lines. TextBox, of course, had all the correct options on.

Serial:

Serial.print(rnd);
Serial.( '\n' );  //carriage return

Following are the ways

1. From the code (the way you have mentioned) ->

   displayBox.Text += sent + "\r\n";
   

   or

   displayBox.Text += sent + Environment.NewLine;
   

2. From the UI
   a) WPF

   Set TextWrapping="Wrap" and AcceptsReturn="True"   
   

   Press Enter key to the textbox and new line will be created

   b) Winform text box

   Set TextBox.MultiLine and TextBox.AcceptsReturn to true
   

Because you haven't specified what front end (GUI technology) you're using it would be hard to make a specific recommendation. In WPF you could create a listbox and for each new line of chat add a new listboxitem to the end of the collection. This link provides some suggestions as to how you may achieve the same result in a winforms environment.